For a long time vehicles have been known which are equipped with a coasting function in order to reduce fuel consumption. This operates in such manner that when the driver throttles down, the driving internal combustion engine is disconnected from the driven vehicle wheels by disengaging a clutch in the drive train. The speed of the combustion engine can then be reduced to an idling speed so that fuel is saved. To reduce the consumption still more, it is also known, in such thrust operation phases of the motor vehicle with the drive power disconnected, to cut off the supply of fuel to the combustion engine completely although, in such a case, more effort is needed for the operation of auxiliary aggregates. In both these operating modes, however, the braking action of the combustion engine, during thrust operation of the vehicle, is not available.
For those familiar with the subject, it is obvious that corresponding to the operating modes for the combustion engine, when driving downhill, the speed of the motor vehicle will increase steadily and, in the absence of an engine braking action, this can only be prevented by actuating a service brake or an additional brake (retarder). In passenger motor vehicles, usually only a service brake associated with the wheels is available so that when such vehicles are driving down steep hills, thrust operation phases with the drive power disconnected are usually avoided.
Further, in the Type FH 12 Volvo® goods vehicle a drive train is known with a drive motor in the form of an internal combustion engine with a torque converter made as an automatic splitter transmission with 12 forward gears with an actuator device to be actuated manually in the form of an accelerator pedal for a power control element (for example, a fuel injection pump) of the drive motor, and with a manually actuated preselector element in the form of a transmission gear range selector lever, located next to the driver's seat, and with a manually operated actuator in the form of a brake pedal for actuating a service brake which decelerates the driving speed of the vehicle.
An electronic control unit, associated with the drive train detects a condition in which no drive power is needed or an engine brake is not engaged; whether an automatic speed control unit is engaged; whether the transmission is in automatic mode, and whether one of the forward gears 7 to 12 is engaged. If all these conditions exist simultaneously, the splitter transmission is automatically shifted to neutral so that the torque-transmitting action, between the combustion engine and the driven wheels, is interrupted.
To overcome the downhill driving problem mentioned earlier, this known coasting function is immediately suppressed by re-establishing the torque flow in the drive train when either the brake pedal or the accelerator pedal is actuated or when the engine brake is engaged.
In addition, in the unpublished patent application DE 10 2005 003 608.2, a method is proposed for operating the drive train of a vehicle, which comprises a drive motor, for example an internal combustion engine, and a torque converter, in the form of an automatic transmission or an automated shift transmission; with a manually operated actuator device for a power control element of the drive motor; with a manually operated preselector element for an operating range of the torque converter or transmission; with a manually operated control element to actuate a brake that decelerates the driving speed of the vehicle, and with an electronic control unit for receiving, processing and emitting signals and data of the drive train and/or of the vehicle. Furthermore, a freewheel device with a coasting function is associated with this drive train.
In order to be able quickly and flexibly, between the conditions “coasting effect permitted” and “coasting effect blocked” by actuating the actuator device for the power control element or by actuating the control element that acts on the brake, the freewheel device is associated with the drive train in such a manner that:    a) in thrust operation when the actuator element for the power control element has not been actuated, the coasting function is at first permitted, and    b) when the control element for reducing the driving speed is then actuated, the coasting function is blocked, and    c) when the coasting function in thrust operation should be permitted again, this is done by operating the actuator device of the power control element.
Furthermore, additional brakes in the practice are known, e.g., hydrodynamic or electrodynamic retarders, which are mainly used in commercial vehicles and which serve to relieve the load on the service brakes associated with the wheels, in that besides the conventional engine brake they have an additional braking action as the result of retarder-internal fluid friction or the production of electric eddy currents. For this, a preselector element, for example in the form of an operating lever, conveys the driver's braking power demand in that, starting from a zero position, the braking resistance increases as the movement of the operating lever is increased or repeated. In combination with an electronic control unit, the retarder can be actuated accordingly.
Regardless of the above, as already mentioned earlier, it can be expedient and advantageous in certain operating situations of the motor vehicle to move the vehicle in the unbraked, rolling condition, whereby fuel can be saved and noise and emissions reduced.
Against this background, the purpose of the invention is to improve upon the prior art by providing the simple and inexpensive possibility, in a drive train of a motor vehicle having an additional brake, in particular a retarder, of explicitly calling for an unbraked rolling condition of the vehicle, also referred to as rolling or coasting operation, and to design the manipulations that produce that operating condition in a manner that is simple and transparent for the driver.